One of the requirements enabling commercial practice of HDTV is the development of magnetic tape capable of high density recording so that numerous digital image signals may be recorded in a compact cassette. Technical Report of Television Society, Vol. 13, No. 59, pages 19-24 (1989) discloses an experiment of digital image recording on magnetic tape having magnetic metal powder coated thereon. Technical Research Report of Electronic Information Communication Society, MR 90-15, pages 39-44, discloses an experiment using Co-Cr evaporated perpendicular magnetic tape.
Magnetic tape having a magnetic layer in the form of a ferromagnetic metal thin film containing a major proportion of Co and a minor proportion of Ni and the like formed by oblique evaporation process are known to have a high saturation magnetic flux density, high coercive force, and improved electromagnetic properties. Technical Research Report of Electronic Information Communication Society, MR 90-7, pages 43-49, discloses a study on the recording mechanism of magnetic tape of the oblique evaporation type by recording rectangular wave signals and analyzing traveling direction dependency and other properties, thereby examining solitary waves reproduced from the magnetic tape. As seen from FIG. 1 on page 43 of this report, the evaporated tape used in this study has a magnetic layer in the form of a monolayer evaporated film which is obliquely evaporated in one direction to allow for unidirectional growth of columnar grains.
In one experiment, solitary wave signals are recorded with optimum recording current flow and then reproduced both while feeding the tape in forward and reverse directions. An observation of four solitary waveforms reproduced in the forward and reverse directions shows that the time taken from the zero cross point to the peak point is less than 1/5 or more than 5 times the time taken from the peak point to the zero cross point as seen from FIG. 4 on page 45. This indicates the appreciable asymmetry of reproduced waveform. The four reproduced waveforms overlap only a little and the overlap of more than zero peaks between the most differing reproduced waveforms is less than 60%. If solitary waveforms reproduced are asymmetric and distorted as mentioned above and waveforms become inconsistent depending on whether recording and reproducing operations are in forward or reverse direction, then actual recording and reproducing operations would result in an increased error rate and a low S/N ratio and require a complex equalizer circuit or even be difficult to equalize. If reproduced waveforms have a degree of distortion, a problem would arise with respect to interchangeability with the so-called metal tape.
Since the commercial standards employ band compression, only a small margin is left for error correction. Then to reduce error rate is an important problem. If the reproduced waveforms are asymmetric, then a width per unit waveform known as "window margin" must be increased in order to suppress the error rate. Such an increased window margin is detrimental to high density recording. A high sampling frequency and a high S/N are not achievable.
It is to be noted that some asymmetry of reproduced waveforms can be corrected by means of an equalizer circuit. But increased asymmetry is quite difficult or impossible to correct. Interchangeability with coating type metal tape is lost. Coating type metal tape provides reproduced waveforms of good symmetry since the easy axis of magnetization is in the plane of the magnetic layer.
Magnetic recording media for digital recording are required to provide not only reproduced waveforms of symmetry, but also a high output because lower outputs result in higher error rates.